Method of operating blast furnaces



Sept. 27, 1938. J. M. AVERY 2,131,031

METHOD 0F OPERATING BLAST FURNAGES Filed June 12, 1936 @fg f6,

ENG/NE 335' Patented Sept. 27, 1938 UNITED STATES PATENT OFFICE METHODOF OPERATING BLAST FURNACES Application June 12, 1936, Serial No. 84,797

15l Claims.

This invention relates to a method of operating blast furnaces forsmelting metals from ores, and has particular reference to the provisionof a method of greatly improving the efficiency of such smeltingprocesses and decreasing the cost of smelting ores in blast furnaces.Although the method of this invention is particularly applicable to andwill be described in connection with the manufacture of pig iron in acoke-fired blast furnace of the conventional type, it is to beunderstnod that the method can be used with equal facility for themanufacture of such analogous products as ferro-alloys, and non-ferrousmetals.

The modern blast furnace has serious inherent and fundamentallimitations, although extensive research and development work in recentyears has brought about considerable improvement in operating efficiencyand facility. The particular fundamental limitations with which thisinvention is chiefly concerned are commonly referred to as solution lossof carbon and "deficiency of hearth heat. By solution loss of carbon ismeant the percentage of carbonaceous fuel which is dissolved in thegases and is accordingly lost. For example, in the i-deal or theoreticalblast furnace, all of the carbonaceous fuel supplied, except thatrequired to carburlze pig iron, would be burned at the tuyres, whereasin actual practice only seventy to eighty per cent of the fuel supply isso burned, the balance being oxidized higher up in the furnace andconstituting the aforementioned loss. This loss results from the factthat in normal blast furnace practice, a considerable proportion of theore reaches high tem- I 35 perature zones deep in the furnace before itis completely reduced, so that when it is reduced in accordance with theslightly exothermic reaction, (l) CO+FeO=Fe+COz, the carbon dioxidewhich is formed immediately reacts with the carbon present in accordancewith the strongly endothermic reaction, (2) CO2-|-C=2CO. Thus, not onlyis carbon lost as fuel at the tuyres, but the gases rising from thehearth of the furnace are cooled by absorption of part of their sensible45 heat, which is required to supply the requirements of reaction (2)and to preheat the charge. Consequently, the solution loss phenomenongreatly increases the fuel requirements of coke per ton vof pig ironproduced, beyond the solution loss 50 per se.

By deficiency of hearth heat is meant that cooling off of the hearthwhich results from blowing the furnace at a blast rate greater than acertain optimum rate, so that when the furnace is operating at no-rmalcapacity it is very sensitive to changes in temperature of the preheatedgases. 'I'he deficiency of hearth heat is probably due to the fact thatthe hearth performs functions Which require that a certain minimumamount of heat developed at the tuyres shall be 5 lat a thermalpotential considerably above an indefinite critical temperature,which'is commonly assumed to be the free running temperature of theslag. This critical temperature has been given as about 1500 C. It hasbeen found l0 that, although this analysis of the condition is generallycorrect, a fundamental cause of the limitation is the inability of theshaft furnace to perform properly its functions of preheating andreducing the charge before it reaches the 15 high temperature zones ofthe furnace. Therefore, if the shaft can be made to perform itsfunctions adequately and low temperature heat thus used to maximumadvantage, the necessity for so large a quantity of high temperatureheat 20 in the hearth is much less urgent.

In ordinary blast furnace practice, the blast gas pressure used is onlythat necessary to force the gas through the stoves and the ducts intothe furnace, through the charge therein and out 25 through the down-takepipe and auxiliary equipment. Nearly all of the pressure drop occurs inthe shaft, and as the pressure at the tuyres is usually between 1A and latmosphere gauge and the top pressure is ordinarily so small as to bepractically negligible, it follows that the average static gas pressurewithin the furnace shaft is between about IA and V2 atmosphere gauge, or11A to 11/2 atmospheres absolute, with the ordinary blast furnaceconstruction and operation. If the blast pressure is increased, a largervolume of blast is forced through the furnace, i. e., the blast rate isincreased, with a resultant increase of dust loss, or of the tendency ofthe charge to hang, or of the hearth to run cold. These are, in fact, 40the conditions which ordinarily limit the blast pressure and hence theblast rate of the ordinary blast furnace, and result in theaforementioned solution loss and hearth heat deficiency.

In accordance with the present invention, a blast furnace for smeltingmetals from ores is artificially maintained and controlled under astatic internal gas pressure which is substantially greater then thepressure developed in the shaft in normal operation, preferably incombination with a material increase in the blast gas feed rate. 'Ihismethod of operation, as will appear hereinafter, radically alters in afavorable manner the thermal balance of the blast furnace and thechemical reactions which take place within it. Thus, under properconditions of operation after the manner of the invention, the solutionloss may be nearly or quite eliminated, the deflciency of hearth heatmay be overcome, the thermal efficiency of the furnace may be increased,and the production capacity of the furnace may be greatly increased.

As will appear from the theoretical discussion hereinafter set forth indetail, if the gases within a blast furnace are maintained undersuperatmospheric pressure after the manner of the invention, and theblast rate is maintained at the same rate as in normal methods ofoperation, solution loss will be practically eliminated and the capacityof the furnace will be increased substantially in proportion tothedecrease in fuel consumption per ton of pig iron. On the other hand, ifthe blast rate is increased in like proportion to the increase ofabsolute average pressure in the shaft, the output ofthe furnace will beincreased in substantially the same proportion and the solution losswill remain substantially unchanged. For most furnaces this would meantoo large a production, and there would be no great saving per ton ofproduct. The invention in its preferred form contemplates increasing theblast rate in about half the proportion of the increase in averageabsolute static gas pressure in the shaft, whereby solution loss iseliminated and at the same time, the capacity of the furnace isincreased not only in proportion to the blast rate, but also by theadded effect of less fuel per ton of product.

In order to carry out the method of this invention in a blast furnace,the shaft is sealed pressure-tightly, the blast gas feed duct and thegas down-take pipe of the furnace are equipped with valve controls, andsuitable blast equipment is supplied whereby the static pressure Withinthe furnace can be built up to some predetermined desired value,preferably between two and seven atmospheres gauge, and thereafterregulated by the valve controls. More particularly, in addition to theaforementioned intake and outlet gas valve controls, the preferredarrangement of the blast furnace and its appurtenant parts for realizingthe method of the present invention, comprises a compressor forincreasing the pressure of the blast gas or gases, a cooler for reducingthe temperature of the compressed blast gas below the dew point, and adehydrator for condensing the moisture therein by precipitation beforesupplying it to the bustle pipe at a predetermined high pressure, whichis regulated and maintained by proper manipulation of the intake andoutlet valve controls of the furnace. The down-take pipe preferablysupplies the combustible blast furnace gas to an internal combustionengine or a compressed gas engine or turbine of suitable form foroperating a compressor positioned in the blast gas line, suitable dustcatchers being provided for removing the dust from the blast furnace gasbefore it is introduced into the engine. If desired, a part of the blastfurnace gas may be supplied to hot blast stoves for preheating the blastin the conventional way instead of being consumed in the 'compressorengine, but one of the principal advantages of the invention is thatpreheating of the blast is in many cases rendered unnecessary.

It will be seen that with a blast furnace provided with the Valvecontrols in the blast feed duct and the gas down-take pipe and suppliedwith high pressure blast gas, the rate of blast passing through thefurnace at a predetermined static pressure may be controlled by suitablevalve manipulation, with the result that the internal static gaspressure is increased several fold, depending upon requirements, sothat, with a blast supplied at a volume greater than the normal volumeof equivalent free air per unit of time, the effect in the furnace isfar reaching in character.

For a more complete understanding of the invention, reference may be hadto the accompanying drawing, in which:

Figure l is a schematic diagram of an arrangement involving theinvention as applied to an existing blast furnace installation, forexample; and

Fig. 2 illustrates a modified arrangement of the auxiliary equipment fora new installation in which advantage is taken of the pressure of theexit gases and the blast air to operate a compressor engine.

Referring to Fig. 1 of the drawing, numeral I0 designates a blastfurnace of more or less conventional design, having a double bell andhopper arrangement II at the upper end of the furnace, which serves asal pressure lock, whereby pressures built up within the shaft of thefurnace I may be maintained during charging operatiolns by propermanipulation of the pressure lock I.

The bustle pipe I2 is also of conventional design supplied by the blastfeed duct I3 which is tted with a control valve I4, in accordance withthe present invention. The air or other feed gas for the blast iscompressed by the compressor I5 to a suitable predetermined pressure,say five atmospheres gauge. The compressed gas is supplied to a suitablecooler I6, which removes the heat of compression therefrom and reducesthe temperature of the compressed gas below the dew point. 'I'hecompressed blast feed gas is then passed through a suitable dehydrator I`I, which condenses the moisture in the gas by precipitation, so that atthe suggested pressure. of five atmospheres gauge, 75% of the moistureis precipitated from air having originally '70% humidity.

'Ihe invention accordingly offers a ready means for drying the blast atpractically no additional cost and very little added equipment. Theadvantages of a dry blast are apparent for with a dry blast thetemperature at the tuyres is considerably higher than with a wet blast.

From the dehydrator, the compressed gas passes directly to the feed ductI3 and through the control valve I4, without preheating, or optionally,it may be preheated by hot blast stoves I8 in the conventional way.

The static pressure within the sealed furnace I0 and the blast ratetherethrough are controllable by means of throttling valve I9 providedin the down-take pipe in accordance with the present invention. Thethrottling valve I9, andthe control valve I4, may be manually operatedor pressure operated, depending upon requirements. Thus, with theassumed compressor pressure of five atmospheres gauge, and allowing fora certain drop due to duct and stove friction, the static pressure inthe shaft can be built up to at least four atmospheres gauge pressure inthe manner described.

The combustible gases generated during smelting in the blast furnace I0are passed under pressure through a suitable dust catcher 2| anddecompressed in a suitable decompressor, such as the turbine 22, thepower output of which may be employed to assist in driving thecompressor I5, the connections being through shafts 23 and differentialgearing 24. l A portion of the combustible gas may be utilized tooperate an internal combustion engine 25 connected through differentialgearing 24 to the compressor I5. A portion of the remaining gas may beled by pipe 28 to the hot blast stoves I8, if preheating of the blastfeed is desired, although that is optional with the present invention.The remainder of the combustible gas may be led by pipe 21 to a boileror the like for generating additional power for operating conveyingequipment or the like.

In cases where new equipment is to be installed, advantage can be takenof the high blast air pressure and the high combustible gas pressure foroperating an internal combustion engine at high efficiency undersupercharged conditions. Such an arrangement is shown schematically inFig. 2, in which the control valves I4' and I9 are employed as before inthe blast air and down-take pipes I3 and 20', respectively. Thecombustible gas is passed under pressure through dust-catcher 2|', and aportion thereof is supplied to the internal combustion engine 25',directly connected by shaft 23' to the blast air compressor I5', whichcompresses the air in the manner described. The pressure air passesthrough cooler I6', dehydrator I1', and, if desired, through blast airpreheating hot blast stoves I8 supplied with the combustiblev gas bypipe 26' from pipe 20'. As aforementioned,'the use of stoves forpreheating the blast air is not essential, but optional.

'Ihe engine 25' is supplied with the compressed air from pipe I3' bypipe 28. so that with the combustible gas and the combustion-supportingair supplied under substantial superatmospheric pressure, the poweroutput economy of the engine is high. The remainder of the combustiblegas is led by pipe 21' to suitable boilers, or the like, for anydesirable purpose.

In operating the blast furnace according to the new method and with theequipment described, the static pressure within the shaft of the furnaceIII is built up by initially closing valve I9 or I9', the blast pressurebeing regulated by control v alve I4 or I4'. The valve I9 or I9' may, ofcourse, be substituted by a valve controlling the flow of the gases tothe turbine, engine or the like. When the gas static pressure within thefurnace has reached the predetermined amount, control valve I9 or I9' isopened manually, or by automatic pressure-controlled'means ofconventional type, by an amount which permits the blast to travelthrough the furnace at the proper predetermined rate, valve I4 in theblast gas feed duct I3 being regulated manually, or automatically bypressure-controlled means, to maintain the proper feed rate of the blastgas through the shaft.

By way of example, it may be assumed that the internal static gaspressure in the shaft I0 is increased fourfold or from four to veatmospheres gauge in this way. Since the specific rate of gas-solidreactions is a direct function of the concentration of gaseousreactants, the specific rate of combustion of the fuel and reduction ofore, are likewise increased fourfold. However, if the blast rate, i. e.,pounds of oxygen fed per minute, remains the same as in normaloperation, the total chemical work done within the furnace per unit oftime remains substantially unchanged. Consequently, the ore is, ineffect, exposed to four times the normal reducing action, which is farmore than enough to insure its complete reduction long before it reachesthe high temperature zones of the shaft. The result is the substantialelimination of solutionlloss of carbon and an approach in this respectto the operation of the ideal blast furnace.

The specific rate of heat transfer from gases to solids in turbulentflow is proportional to the mass velocity of the gas up to pressures ofat least thirty atmospheres. Since, in the assumed example, the blastrate is normal, the specific rate of heat transfer likewise remainsnormal, and as the total transfer of heat to be eifected per unit oftime is therefore practically the same as in normal operation, it mightbe supposed that efflciency of h`eat exchange between the gases and thesolid charge would not be affected. However, in most blast furnaceoperations, this exchange of heat is very seriously affected bychanneling in the charge due to high gas velocities, and I have foundthat the much lower gas velocities of the example cited, i. e.one-fourth the normal velocity, practically eliminate such channeling.The result is a substantially complete exchange of heat between thegases and the charge, which insures that much of the sensible heatordinarily lost in the stack gases is conserved by being utilized topreheat the charge.

Similarly, the pressure drop of gases flowing through heterogeneouslypacked towers for a given rate of flow or mass velocity varies inverselyas the overall pressure. The pressure drop through the furnace withnormal blast rate and four times normal pressure according to thepresent invention will consequently be about onefourth atmosphereinstead of about one atmosphere, which has an important eifect upon theamount of power required to compress the blast, and upon the tendency ofthe charge to hang. In the ordinary blast furnace the normal pressuredrop is about one atmosphere with a blast pressure of two atmospheresabsolute. Therefore, to quadruple the average absolute pressure in theshaft, as compared with normal blast furnace operation, i. e.. in orderto produce an average absolute static pressure of 4X l1/2=6 atmospheresin the shaft, the blast pressure required would be, not 4 2=8, but6+1A1=6l1 atmospheres absolute, provided the normal blast rate is usedat the increased pressure.

'I'he use of pressure has further advantages. For example, thesubstantial elimination of solution loss and increased efficiency in theuse of heat, make it possible, other conditions being constant, toconsiderably decrease the ratio of coke to ore, i. e., to increase theburden of the furnace. Since, in the assumed example, the blast rate istaken as normal, coke will be burned at the normal rate at the tuyresand the throughput of the furnace will therefore increase in proportionto the increased burden. It has been found that under such conditions,using normally preheated blast, the capacity of a furnace rated at 400tons a day, for instance, will be increased to as much as 500 tons aday, while the coke required will be decreased from about 2000 pounds toas little as 1460 pounds per ton of pig iron.

There is, however, an upper limit to the permissible pressure, set bythe equilibrium conditions of the reaction 2CO=CO2+C which is drivenstrongly from left to right by increased pressure, and which is stronglycatalyzed by metallic iron and iron ore. Thus, a mixture of CO and CO2in equilibrium with excess carbon at 900 C..wil1 contain about 3% CO2 atone atmosphere total pressure, and about 14% CO: at six atmospherestotal pressure. At lower temperatures the effect'is still morepronounced; at '700 C. the corresponding figures being 38% CO2 and 68%CO2 respectively. In the blast furnace the partial pressures of thegases are greatly decreased by dilution with nitrogen, but therenevertheless remains a limitation in pressure which it is not desirableto exceed because of this side reaction. I have found that at overallpressures of about ten atmospheres gauge this effect may become serious,and for this and other reasons I prefer to carry out the method of theinvention at lesser pressures, namely, from two to seven atmospheresgauge.

It has been ascertained that under the conditions assumed in theexample, the amount of high temperature heat available in the hearth isfar more than is required, instead of less as is usual. The excess ofhigh temperature hearth heat is, in fact, so great that it becomespossible to use cold blast together with a larger proportion of fuel,with the result that the throughput is decreased 7%, and the amount ofcoke required per ton of pig iron decreased 4%, as compared with normaloperation. 'Ihis small decrease in capacity is far more than offset bythe saving in coke, elimination of preheating stoves, and the diversionto useful purposes of the fuel gas normally required to preheat theblast.

Thus far it has been assumed that the blast rate is maintained at thenormal rate. If, however, the 'blast rate is assumed to be increased inproportion to the increase in pressure, it is apparent that the totalchemical work effected within the shaft per unit of time will likewiseincrease in substantially the same proportion, from which it followsthat the thermal balance of the furnace, and the solution loss of carbonwill be much the same as in present methods of operation. There istherefore a very definite upper limit of blast rate beyond which thepeculiar advantages of operation under relatively high static pressurecease to exist, and that limit is a ratio of increase in blast ratesomewhat less than the ratio of pressure increase. It seems; to

be generally agreed that, for reasons not connected with actual thermaland chemical conditions within the furnace, the practical limit of blastfurnace capacity has been reached at about 1000 tons of pig iron perday, and as modern furnaces have an average capacity on the order of 400to 500 tons a day, there would seem to be only moderate advantage in theuse of pressure merely as a means of increasing the capacity of afurnace. But, as previously pointed out, if the average absolute staticpressure Within the shaft is increased for example fourfold, i. e., toabout 6 atmospheres absolute, and the blast rate is increased, but insubstantially smaller proportions, for example twofold, the results uponfurnace operation and economy are far reaching. Elimination of solutionloss and greater thermal eillciency increases the furnace capacity perunit blast rate by about 20%, and this coupled with the assumed doublingin blast rate increases the furnace capacity by a factor of about 2.5 ascompared with normal operation. The invention in its preferred formtherefore contemplates maintaining within the shaft of the blast furnacean average absolute static gas pressure of about four times the normalpressure, and increasing the blast rate in proportion to about half theincrease in absolute static gas pressure within the shaft.

In view of the large amount of air required for the blast it is apparentthat the power and equipment required to compress the blast to therelatively high pressures of the method of the invention must becarefully considered. It has been found that, contrary to offhandjudgment, high blast pressure may actually result in a decrease in thenet amount of power required for blowing. If the average internalfurnace pressure is quadrupled, the blast pressure must be increasedfrom between about one-half and one atmosphere gauge to between aboutfour and five atmospheres gauge, which requires about three times thenormal amount of power per unit of blast rate. In the case of normalfurnace operation practically all of the power used to compress theblast is lost by friction of the gas in passing through the furnace andthe gas leaves the furnace at substantially atmospheric pressure. But inthe method of the invention the pressure drop through the furnace isonly a fraction of one atmosphere, so that in the example given the gasleaves the furnace at approximately the static internal pressure.Consequently, the blast furnace gas may be expanded through a turbine orreciprocating compressed gas engine whereby some 60% of its energy ofcompression may be recovered. Moreover, the heat of compression of theblast corresponds to a temperature rise of about 400 F., and a largepart of this heat may be recovered by heat exchange. In this mannerabout 80% of the power originally required to compress the blast may bereconverted into mechanical energy and used to compress the blast.

It is thus clear that the net power which must be used for compressingthe blast is, in the preferred method of the invention, on the order ofhalf that required in normal blast furnace operation, per unit of pigiron produced. In the case of existing installations the presentequipment may be used for preliminary compression, and additionalequipment provided merely for raising the blast from the presentpressure to the desired pressure. It will be evident that in blastfurnace operation after the manner of this invention, the blast can bedehydrated without extra expense, the blast in most cases need not bepreheated, solution loss is practically eliminated, the capacity of afurnace of given size can be increased several fold, and most of theenergy required for blast compression can be recovered and usefullyemployed.

The method of the invention accordingly provides means whereby longrecognized diflculties and limitations relating to the manufacture ofpig iron and ferroalloys in blast furnaces may be largely overcome. Bysubstantially eliminating solution loss of carbon and correcting thedeficiency of hearth heat, the method greatly increases the capacity ofa furnace of given size, decreases the ratio of fuel required per ton ofproduct, and nearly or quite eliminates the necessity of preheating theblast. It simplifies the problems of smelting ne or improperly sized ordifflcultly reducible ores, and eliminates many of the causes ofirregularities of furnace operation. Its beneficial effects lead to asubstantial saving in fuel both for smelting the ore and for preheatingthe blast, in the net amount of power required to compress the blast,and in labor costs and fixed charges. In the case of new plants, itgreatly decreases the capital investment required for a given capacity.

Although the invention has been described in a sealed furnace shaftcapable of operation atv ferroalloys, and has perhaps greatest utilityin that field, it'is also applicable to smelting those non-ferrousmetals which do not volatilize under conditions of operation describedherein. Also, the term throttling, as employed herein and in appendedclaims, comprehends within its scope the retardation, by any means, ofthe furnace discharge gas, whereby a back pressure is created.

v1. The method of operating a. sealed blast furnace, which comprisessupplying the blast feed gas at superatmospheric pressure, andthrottling the gas discharge from the furnace to maintain an averagestatic internal pressure of between about two and seven atmospheresgauge.

2. 'I'he method of operating a sealed blast iurnace, which comprisessubjectingthe ore therein Y to an average static pressure of between twoand seven atmospheres gauge by maintaining the blast gas feed pressureabove about two atmospheres gauge and controlling the flow thereofthrough the furnace and ore to avoid a pressure drop in excess of aboutone atmosphere.

3. The method of decreasing the deficiency of hearth heat in a blastfurnace during operation, which comprises maintaining the pressure ofthe blast feed gas at the hearth in excess of a normal blast pressure ofbetween about one and one-half and two atmospheres absolute, andregulating the rate of flow of the blast gas through the furnace bythrottling the discharge of the gas from the furnace to create a staticpressure in the furnace of between about three and eight atmospheresabsolute.

4. The method of operating a sealed blast furnace, which comprisescompressing the blast feed gas in excess of two atmospheres absolute andnot materially in excess of about eight atmospheres absolute, throttlingthe discharge of the gas from the furnace to prevent a pressure dropexceeding about one atmosphere to maintain the average static pressurewithin the furnace in excess of about two atmospheres absolute, andpreheating the blast feed gas after compression.

5. The method of smelting ore in a blast furnace, which comprises`maintaining a blast feed gas pressure between about two and sevenatmospheres gauge in the furnace, and throttling the discharge of thegas from the furnace to maintain the average static pressure within thefurnace in excess of two atmospheres absolute and to increase the blastrate through the furnace by an amount not materially in excess of aboutonehalf the blast rate produced with unthrottled gas discharge.

6. In a blast furnace having a blast gas feed pipe and a gas dischargepipe, the combination of internal pressures in excess of about twoatmospheres gauge, means for compressing the blast feed gas supplied tothe feed pipe to a pressure in excess of two atmospheres absolute, andthrottling means in the gas discharge pipe for building up the pressurewithin the furnace in excess of two atmospheres gauge.

'7. In a blast furnace having a blast feed pipe anda gas discharge pipe,the combination of a sealed furnace shaft capable'"of operation atsuperatmospheric pressure, a compressor for compressing the blast feedair supplied to the feed pipe to a pressure in excess of two atmospheresabsolute for maintaining the pressure Within the furnace and thedischarge therefrom at not in excess of about eight atmospheresabsolute, an

internal combustion engine, connections between the gas discharge pipeand the engine for supplying at least a part of the discharge gaslasfuel to the engine at said pressure in excess of two atmospheresabsolute, means for supplying a part of the blast feed air to the engineas combustionsupporting air at said pressure in excess of twoatmospheres absolute, and driving connections betwen said engine andsaid compressor.

8. The method of increasing the output, decreasing the solution loss,and increasing the quantity of hearth heat available in a blast furnaceduring operation, comprising maintaining the blast pressure at a gaugepressure substantially greater than the normal gauge blast pressure ofabout one-half to one atmosphere and not materially in excess of sevenatmospheres, and throttling the discharge of gases from the furnace tocontrol the blast rate to a rate not greater than about one-half therate obtained by allowing the gas to escape Without throttling.

9. The method of increasing the output, decreasing the solution loss,and `increasing the quantity of hearth heat available in a blast furnaceduring operation, comprising maintaining the blast pressure at a gaugepressure substantially greater than the normal gauge blast pressure ofabout one-half to one atmosphere and not materially in excess of sevenatmospheres, and throttling the discharge of gases from the furnace tocontrol the blast rate to a rate substantially less than the normalblast rate multiplied by the ratio obtained by dividing the absoluteblast pressure employed, by the normal absolute blast pressure.

10. The method of increasing the output, decreasing the solution loss,and increasing the quantity of hearth heat available in a blast furnaceduring' operation, comprising maintaining the blast pressure at a gaugepressure substantially greater than the normal gauge blast pressure ofabout one-half to one atmosphere and not materially in excess of sevenatmospheres,

' and throttling the discharge of gases from the furnace to control theblast rate to a rate substantially equal to the normal blast rateproduced by allowing the gas 'to vescape without throttling multipliedby one half the ratio obtained by dividing the absolute blast pressureemployed, by the normal absolute blast pressure.

11. The process of operating a blast furnace to reduce solution loss offuel, increase output and the quantity of hearth heat available in theblast furnace during operation, comprising maintaining the blastpressure at a gauge pressure in excess of two atmospheres, andthrottling the discharge of gases from the furnaceto maintain a pressuredrop through the furnace not greater than about one atmosphere and anaverage static pressure in the furnace of between about two and aboutseven atmospheres gauge.

l2. The process of operating a blast furnace to reduce the solution lossof fuel, increase output and the quantity of hearth heat available inthe blast furnace during operation, comprising maintaining the blastpressure while throttling the discharge of gas from said furnace tomaintain an average gauge pressure in said furnace of between about twoand about seven atmospheres, and a blast rate substantially less than ifthe discharge gas were allowed to escape without throttling,

13. The process of operating a blast furnace to decrease solution lossand increase output per unit of blast rate, which comprises maintaininga blast pressure in excess of a normal blast pressure of about one-halfto about one atmosphere gauge, and throttling the discharge of gas fromthe furnace to create an average static pressure in the furnace notmaterially exceeding seven and less than ten atmospheres gauge.

14. The process of operating a blast furnace to increase output anddecrease solution loss, which comprises maintaining a blast pressure inexcess of a normal blast pressure of about one-half to about oneatmosphere gauge, and throttling the discharge of gas from the blastfurnace to create a static pressure in the furnace of less than tenatmospheres gauge and to maintain a. blast rate substantially less thanwould result by allowing the gas at a normal shaft pressure of betweenabout one-fourth and onehalf atmosphere gauge to escape withoutthrottling.

15. The process of operating a blast furnace to increase output anddecrease solution loss, which comprises maintaining a blast pressure inexcess of a normal blast pressure of about one-half to about oneatmosphere gauge, throttling the discharge of gas from the furnace tocreate an average static pressure in the furnace not materiallyexceeding seven atmospheres gauge and to maintain a blast rate notmaterially in excess of one-half the rate that would result if the gaswere allowed to escape without throttling.

JULIAN M. AVERY.

